Scientists from the U.S. Centers for Disease Control on Monday reported further evidence that there are now two distinct subgroups of the H5N1 avian flu virus that have infected humans. Previous research established that the first subgroup, or clade, appeared in 2003 and sickened people in Vietnam, Cambodia and Thailand. The second clade showed up in 2005 in Indonesia.

Fortunately, both clades still lack the ability to pass easily from one person to another. But this dose of double trouble further complicates research on any potential avian flu vaccine, which must now take both groups of strains into account.

Update (March 29, 2006): You can read more entries on avian flu here (an interview with a survivor of the 1918-1919 pandemic), here (how Google Earth is helping poultry veterinarians prepare) or here (a collection of my avian flu posts on Technorati).

—Christine Gorman